This invention relates to the measurement of the void fraction in two-phase flows.
The void fraction in a material is defined as the ratio of the volume of gas to a total volume under consideration that includes the gas. The term "void fraction" has also been applied to give a measure of the amount of gas present in a mixture of a gas and particulate matter such as is found in a fluidized bed. That use will be excluded from our description of the term since the consideration here is limited to the void fraction in a flowing liquid containing a gas. The word "local" is also used to refer to measurements that are made in a region that approximates a point in contrast to the term "global" which provides a measure of the average ratio of gas to liquid. It can be expected that local measurements of the void fraction in a mixture of a liquid and a gas will provide information about the sizes of voids and their spatial distribution in the fluid. Two-phase flows of the type described here are commonly encountered in applications involving liquid-metal magnetohydrodynamics and also in experiments connected with development of liquid-metal fast-breeder reactors. Some of the combinations of liquids and gases that have been used in two-phase systems are mixtures of NaK and nitrogen, liquid tin and steam, and air and water.
Many methods have been used in the past to measure void fraction. A summary of the present state of the art is given in a report entitled "Experimental Methods in Two-Phase Flow Studies," prepared for the Electric Power Research Institute and published in EPRI NP-118, dated March 1976. Methods in use include a measurement of the attenuation of a beam of X-rays or gamma rays that is passed through the flowing stream. It is often useful to measure the flux of gamma rays with the flow conduit full of the gas and then measure with the conduit full of the liquid to establish the extremes of attenuation and to derive or infer a relationship to associate particle density in the beam with the global void fraction. Another method of determining void fraction is to calculate it from the measurement of a pressure drop between two locations in the stream. This also is a global method. A third method, the hot-film probe, makes use of the fact that the heat transfer from a hot-film probe into a flowing liquid stream is different when the liquid contains bubbles. The hot-film probe makes local measurements. Another local method is the optical probe which passes light down a probe to its tip which is exposed to the fluid stream. Some light is reflected back from the tip. The presence of a gas void at the tip of the optical probe varies the index of refraction at the tip and hence affects the light that is reflected back from the probe. Other methods of measuring the void fraction include the use of quick-closing valves to trap a sample in a flow channel and determine how much of the trapped sample is liquid and how much is gas. This method and most of the photographic methods that make use of changes in density or other properties of the fluid are also global methods. Of the local methods, the hot-film probe requires more costly and complicated instrumentation and is affected by changes in the temperature of the fluids in which the void fraction is to be measured. The optical probe can only be used in a limited number of fluid-gas combinations.
An impedance-variation probe has been used with success to measure the local void fraction in various two-component streams at low velocities. The impedance-variation probe connects electrically to the fluid stream and uses the variation in impedance between the probe tip and a fluid ground to detect the presence of a gas void or a continuum of liquid at the tip. When bubbles contact the impedance-variation probe, the current flow is affected by the change in impedance or resistance that is represented by the bubble. With impedance-variation probes now in use, the conducting tip is of the order of 0.025 to 0.25 mm. The use of such probes has been hampered in the past by the fact that the combination of distributed capacitance of the probe and the desirability of having a small conducting probe area to approximate point measurements leads to a relatively long time constant. It is evident that, if the combination of void size and stream velocity is such that the time necessary for a complete void to pass the tip of a probe is not considerably greater than the RC product of the tip, then the probe will require special treatment to resolve two adjacent voids, or even to detect a small, fast-moving void at all.
It is an object of the present invention to provide a better apparatus for measuring the void fraction in a fluid stream made up of a liquid and a gas.
It is a further object of the present invention to provide an apparatus including an impedance-variation probe and an electronic circuit to resolve and identify small voids in a fluid stream including a liquid and gas-filled voids.
Other objects will become apparent in the course of a detailed description of the invention.